1. Field of the Invention
The present disclosure generally relates to semiconductor devices. More specifically, the present disclosure relates to tunneling field-effect transistors.
2. Description of the Related Art
Advances in the semiconductor industry have reduced the size of transistors in integrated circuits (ICs) to 45 nm. Continuing pressure to create smaller and more power efficient products will continue to reduce the transistor size to 32 nm and smaller. Decreases in transistor size lead to decreases in power supply voltage to the transistors and capacitance of the transistors. As the power supply voltage has decreased, the threshold voltage of the transistors in the ICs has also decreased.
Lower threshold voltages are difficult to obtain in conventional metal-oxide-semiconductor field-effect transistors (MOSFETs) because as the threshold voltage is reduced the ratio of on current to off current (Ion/Ioff) also decreases. The on current refers to the current through a MOSFET when a gate voltage applied is above the threshold voltage, and the off current refers to current through a MOSFET when a gate voltage applied is below the threshold voltage.
Tunneling field-effect transistors (TFETs) have improved Ion/Ioff ratios. Band-to-band tunneling in TFETs increases the achievable Ion allowing further reductions in threshold voltage, power supply voltage, and transistor size. A conventional TFET includes a drain region and a source region in a substrate layer and the drain region and the source region are doped with opposite carriers. For example, the drain region may be an n-doped region and the source region may be a p-doped region. A gate oxide is deposited on the substrate layer, and a gate electrode is deposited on the gate oxide. A gate voltage above the threshold voltage applied to the gate electrode switches the TFET from an off state to an on state.